Currently, 3rd generation cellular communication systems are being rolled out to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. There may also be a time division multiple access (TDMA) component in CDMA systems, where user separation is also achieved by assigning different time slots to different users.
In FDD systems, uplink and downlink communication occur on separate carriers. Uplink transmissions are those from the mobile wireless communication unit (often referred to as wireless subscriber communication unit) to the communication infrastructure via a wireless serving base station. Downlink transmissions are those from the communication infrastructure to the mobile wireless communication unit via a serving base station. In contrast to FDD systems, TDD systems use the same carrier frequency for both uplink and downlink transmissions. In both FDD and TDD systems, the carrier frequency may be subdivided in the time domain into a series of timeslots in order to provide a TDMA component. For TDD, the single carrier frequency is assigned to uplink transmissions during some timeslots and to downlink transmissions during other timeslots. For FDD, a carrier frequency operable in either uplink or downlink mode may service different users during different time regions, which may comprise one or more timeslots. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of UMTS, can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a conventional cellular system, cells in close proximity to each other are allocated non-overlapping transmission resources. For example, in a CDMA network, cells within close proximity to each other are allocated distinct spreading codes (to be used in both the uplink direction and the downlink direction). This may be achieved by, for example, employing the same channelisation spreading codes at each cell, but a different cell specific scrambling code. The combination of these leads to effectively distinct spreading codes at each cell.
A typical and most cost-effective approach in the provision of multimedia services is to ‘broadcast’ (point-to-multipoint transmission) the multimedia signals, as opposed to sending the multimedia signals in an uni-cast (i.e. point-to-point) manner. For broadcast point-to-multipoint transmission, a single carrier frequency conveys broadcast information to a plurality of wireless subscriber units from one wireless serving communication unit, i.e. one cell. Typically, tens of channels carrying say, news, movies, sports, etc. may be broadcast simultaneously over such a communication network. Conversely, for uni-cast operation, the communication is on a one-to-one basis between a wireless subscriber unit and a serving wireless communication unit, i.e. the information conveyed is unique to one wireless subscriber unit.
In some cases, an entire carrier may be dedicated to the sending of broadcast or point-to-multipoint information. The broadcast carrier may be associated with one or more uni-cast communication channels, which may be operable over one or more separate carrier frequencies. Additionally, it is possible for both uni-cast and point-to-multipoint broadcast traffic to be conveyed over the same carrier frequency but typically at different times. In general, uni-cast communication may allow for the establishment of security and authentication mechanisms related to the communication of the broadcast information between a wireless subscriber unit and a communication network, and may also facilitate the transfer of broadcast service information to the wireless subscriber unit. Other user-specific communication may be performed over the uni-cast carrier(s), which may or may not relate to the operation of broadcast services on the same or another carrier frequency.
Digital communication systems may use so-called non-coherent or coherent signalling methods. For either method, it is common that the transmitting entity maps the desired bit sequence for transmission onto a sequence of modulation symbols, each adopting one of a finite alphabet of symbols or waveforms. As the signals propagate from the transmitter to the receiver, the phase of the transmitted signal varies in space and in time. Generally, at a receiver, the phase of the received signal is arbitrary.
In the non-coherent method, the receiver does not require knowledge of the phase of the received signal in order to demodulate the signal and to recover the transmitted data. That is, the members of the transmitted symbol alphabet for the non-coherent modulation scheme may be distinguished from one another by the receiver without the need for absolute phase information.
Conversely, for coherent modulation schemes, members of the transmitted symbol alphabet may appear similar to one another at different phases. Thus, for these schemes, it is imperative that the receiver is able to determine the received phase of the signal in order to distinguish between the received symbols and to correctly recover the data. In many circumstances, coherent modulation schemes are able to carry spectral efficiency advantages. Hence, coherent modulation schemes are commonly used for high-capacity digital communication and broadcast systems.
In the coherent scheme, the transmitter often sends a reference signal along with the transmitted data. The receiver has a-priori knowledge of the structure of this reference signal. Hence, the receiver is able to look for the presence of the reference signal within the received signal. Upon finding the reference signal, the receiver may determine its amplitude and phase and, assuming that both the reference signal and the communication data have passed through the same propagation channel between transmitter and receiver, the phase of the additionally received communication data symbols is then also known and the modulation symbols may be recovered. The process of estimating the amplitude and phase of the radio propagation channel in the receiver is known as ‘channel estimation’.
The reference signal is often referred to as a ‘pilot’. At the transmitter, the pilot must be multiplexed with the data in some way such that both may then be carried over the communication link to the receiver, with the intention of ensuring that both will experience the same or similar phase adjustments by the time that the signals arrive at the receiver. Code-Division Multiplexing (CDM), Frequency-Division Multiplexing (FDM) and Time-Division Multiplexing (TDM) methods are each individually used in various communication systems to transmit pilot signals as well as data.
FIG. 1 illustrates examples of these pilot/data multiplexing possibilities. For example, a first graph 100 illustrates a CDM technique with the code values 115 plotted against time. Here, it is shown that the data is sent using a first set of codes and the pilot signal is sent using a second or second set of codes. A second graph 150 illustrates a TDM technique with a code or frequency 155 plotted against time. Here, the data 165 is sent in a first time period with the pilot signal 170 sent in a second time period.
The current UMTS WCDMA FDD system utilises CDM between the pilot signal and the data. The pilot is termed the Common Pilot Channel (CPICH). The CPICH is designed such that it is orthogonal in the code domain to the data. This helps to reduce interference between the data and pilot signal, which is beneficial in terms of receiver performance. The presence of code-domain orthogonality between the pilot signal and data helps to avoid the possibility of the data signal interfering with the pilot, which would otherwise reduce the quality of the estimate of the amplitude and phase of the pilot. This means that the characteristics of the radio propagation channel can be better-ascertained by the receiver and demodulation performance is improved (such as through a reduced number of communication errors, improved geographical coverage of the system, improved communication data rates, etc.).
The code-domain orthogonality between the pilot and data is present at the transmitting side, but can sometimes be degraded or destroyed by the time that the signals arrive at the receiver. This degradation is often due to the action of the intervening radio propagation channel. In particular, radio channels with a large amount of signal dispersion may significantly degrade the degree of orthogonality between pilot and data using a CDM technique. An example of this signal dispersion (a spreading in time of the signal energy due to multiple reflections and the differing path lengths of individual propagation rays) is illustrated in the time dispersion graphical representation 200 of FIG. 2. Thus, as shown, in some radio environments, a code domain pilot is susceptible to the radio propagation channel and exhibits a variable channel amplitude (and also phase—not explicitly shown) response 210 over time 215. As such, the use of CDM pilot signals can be less effective than would be desirable.
In such scenarios, it can be beneficial to alternatively utilise time division multiplexing for the pilot signal and data. Orthogonality between the pilot signal and the data is again susceptible to degradation due to the overlap of energy between the two, which is caused by the delay spread in the radio propagation channel. Referring back to FIG. 1, the time dispersion of data 175 in the radio channel may lead to a time domain overlap of energy between data and pilot signals. In the subsequent region 180, the pilot signal is not affected by the data even in the presence of dispersion, due to the time limitation of the imposed dispersion. Hence, the quality of the channel estimation is not degraded if this portion of the pilot signal is used for channel estimation.
Therefore, by allowing for some guard separation in the time domain between the pilot signal and data, or by careful design of the TDM pilot sequence, it is still possible to receive a portion of the TDM pilot that is unaffected by the data (and vice versa). Such careful design may ensure accurate estimation of the amplitude and phase of the radio channel using the TDM pilot as well as deliver an improved demodulation performance. As previously stated, these improvements in demodulation performance may be translated into system gains such as improved geographical coverage or increased data rates.
Large amounts of signal dispersion can occur due to the presence of multiple reflections in the radio channel. Larger differential path delays lead to a larger extent in time of the dispersion whilst the presence of multiple reflectors leads to an increased number of components (more paths). Such channels are referred to herein as ‘complex’ radio channels in that they may exhibit a large number of reflections.
One particular scenario where complex radio channels may be observed is that of the Single Frequency Network (SFN) transmission method for broadcast. In this transmission method, the same data is transmitted using the same signal waveforms from multiple transmission sites (i.e. multiple communication units) in a synchronised manner. The waveforms travel towards the (potentially mobile) receiver (i.e. wireless subscriber unit) and experience differing delays and amplitude and phase adjustments as they do so. The signals combine in space sometimes constructively and sometimes destructively. The presence of differing signal delays can allow for signals with a path delay difference to be resolved and constructively combined by the receiver. This process is often referred to as equalisation. Accurate channel estimation is therefore essential in such systems and environments to enable the constructive combination of the signals from the multiple transmission sites. In the absence of equalisation, the presence of multiple path reflections may severely degrade the radio link quality.
Therefore, it has been determined that the use of a CDM pilot for systems (such as the aforementioned SFN broadcast system) may not be overly appropriate in complex propagation channels. The use of a TDM pilot may offer advantages in terms of channel estimation and receiver performance. However, many 3GPP receivers are designed to operate using CDM pilots (and may also use the pilot for purposes other than channel estimation in the receiver).
Consequently, current techniques are suboptimal. Hence, an improved mechanism to address the problem of pilot transmission schemes, for example over a broadcast cellular network, would be advantageous.